This invention relates to the scarfing of metal workpieces wherein defects on the surface of a workpiece are removed by a stream of scarfing oxygen gas. More specifically, this invention relates to a method and apparatus for spot scarfing one or more areas of the workpiece surface while the scarfing apparatus and workpiece are in motion relative to each other at normal scarfing speed.
Scarfing cuts are conventionally started by first preheating a band of metal on the surface of a workpiece to its oxygen-ignition temperature. The width of the band is normally equal to the width of the desired scarfing cut. A stream of scarfing oxygen is then impinged upon the preheated band and relative movement is provided between the scarfing oxygen stream and the workpiece, thereby producing the desired scarfing cut. During the preheating step of such a conventional process, which may take as long as 20 seconds or more, there can be no relative motion between the workpiece and the scarfing apparatus, since relative motion would prevent the band from becoming preheated to the required temperature. This period during which there can be no relative motion makes such conventional starts impractical for spot scarfing individual defects, because the frequent need to stop the relative motion for preheating would cause the spot scarfing operation to take too long if several defects had to be scarfed. Furthermore, if several scarfing units are arranged side-by-side to make a gang pass over the surface of the workpiece, stopping relative motion to make a start with one unit, while another unit is making a scarfing cut, would cause the scarfing stream of the unit making the cut to gouge an unacceptably deep groove in the workpiece during the period of no relative motion.
U.S. Pat. Nos. 3,991,985 and 3,996,503, as well as 4,038,108 describe methods and apparatus for instantaneously starting a scarfing reaction without stopping for preheating. While these inventions represent significant advances in the art, they both have the disadvantage of requiring relatively expensive and complicated equipment. The apparatus of U.S. Pat. Nos. 3,991,985 and 3,996,503 requires a wire feeding mechanism, and that of U.S. Pat. No. 4,038,108 requires a laser. Moreover, both of the inventions require starting a new reaction each time a scarfing cut is to be made, thereby necessitating frequent use of the wire feeder or laser.